1. Field of the Invention
The invention relates generally to downhole tools for well logging. More particularly, the invention relates to improved designs of downhole tools to facilitate a logging system to adapt to different situations.
2. Background Art
Oil and gas industry uses various tools to probe the formation penetrated by a borehole in order to locate hydrocarbon reservoirs and to determine the types and quantities of the hydrocarbons. These tools may be used to probe the formations after the well is drilled, i.e., wireline tools. Alternatively, these tools may be included in a drilling system and make measurements while drilling, i.e., measurement-while-drilling (MWD) tools or logging-while-drilling (LWD) tools. In addition, measurements may also be made while the drill string is being tripped out of the well, i.e., logging-while-tripping (LWT) tools. The difference between the MWD and LWD tools is not germane to the present invention. Thus, in the following description, LWD will be used to generally include these two different types of operations.
FIG. 1 shows a general illustration of a drilling rig and an LWD tool in a borehole. The rotary drilling rig shown comprises a mast 1 rising above ground 2 and is fitted with a lifting gear 3. A drill string 4 formed of drill pipes screwed one to another is suspended from the lifting gear 3. The drill string 4 has at its lower end a drill bit 5 for the drilling well 6. Lifting gear 3 consists of crown block 7, the axis of which is fixed to the top of mast 1, vertically traveling block 8, to which is attached hook 9, cable 10 passing round blocks 7 and 8 and forming, from crown block 7, on one hand dead line 10a anchored to fixed point 11 and on the other active line 10b which winds round the drum of winch 12.
Drill string 4 is suspended from hook 9 by means of swivel 13, which is linked by hose 14 to mud pump 15. Pump 15 permits the injection of drilling mud into well 6, via the hollow pipes of drill string 4. The drilling mud may be drawn from mud pit 16, which may be fed with surplus mud from well 6. The drill string 4 may be elevated by turning lifting gear 3 with winch 12. Drill pipe raising and lowering operations require drill string 4 to be temporarily unhooked from lifting gear 3; the former is then supported by blocking it with wedges 17 in conical recess 18 in rotating table 19 that is mounted on platform 20, through which the drill string passes. The lower portion of the drill string 4 may include one or more tools, as shown at 30, for investigating downhole drilling conditions or for investigating the properties of the geological formations. Tools 30 shown may be any type of tools known in the art.
Variations in height h of traveling block 8 during drill string raising operations are measured by means of sensor 23 which may be an angle of rotation sensor coupled to the faster pulley of crown block 7. Weight F applied to hook 9 of traveling block 8 may also be measured by means of strain gauge 24 inserted into dead line 10a of cable 10 to measure its tension. Sensors 23 and 24 are connected by lines 25 and 26 to processing unit 27 which processes the measurement signals. Recorder 28 is connected to processing unit 27, which is preferably a computer.
Different tools (shown as 30 in FIG. 1) used in formation logging are often based on different sensor technologies for probing different formation properties. For example, resistivity tools may be used to measure formation conductivity or its inverse, resistivity. Such tools include, for example, Formation MicroScanner/MicroImager sold under the trade name of FMS/MI™ and Oil-Based Mud Imager sold under the trade name of OBMI™ from Schlumberger Technology Corp. (Houston, Tex.). FMS/MI™ is a wireline tool for use in water based mud (WBM), while OBMI™ is a wireline tool for use in an oil-based mud (OBM). For description of a tool like OBMI™, see U.S. Pat. No. 6,191,588 B1 issued to Chen and assigned to the assignee of the present invention.
Other types of resistivity tools may include Resistivity-at-bit (RAB™ from Schlumberger Technology Corp.) and GeoVision Resistivity (GVR™ from Schlumberger Technology Corp.). These tools (RAB™ and GVR™) are LWD tools for use in a water-based mud (WBM); they use current injection to probe the resistivity of formations. For description of the working principles of the RAB™ and GVR™ tools, see U.S. Pat. No. 5,235,285 issued to Clark et al. and assigned to the assignee of the present invention.
In addition to resistivity, other formation properties commonly logged for oil and gas exploration include formation density, formation porosity, formation sedimentation structures, etc. These other formation properties may be logged with ultrasonic energy, gamma radiation, neutron radiation, or nuclear magnetic resonance, to name a few. Ultrasonic Borehole Imager (UBI™ from Schlumberger Technology Corp.) is a wireline tool that uses ultrasonic echo pulses for the measurements. Azimuthal density neutron tool (ADN™ from Schlumberger Technology Corp.) and vision density neutron tool (VDN™ from Schlumberger Technology Corp.) are LWD tools that use neutron radiation to probe formation density.
Complexity of formation logging arises not only from diverse tools based on different working principles, but also from different requirements that may depend on, for example, geology, drilling practices, and client priorities. Furthermore, the different requirements may also be dictated by different muds, different formation properties of interest, different ranges of values of the formation properties, and different requirements for accuracy and resolution.
To minimize the time and cost of a logging operation, an assortment of tools, if they are compatible, may be attached to a single logging system. Otherwise, a logging operation may require multiple runs. In order to increase the efficiency and to reduce the cost of a logging operation, it is desirable that various tools, sensors, and their components be readily interchangeable and similar components can be readily shared with different tools.